Magnetic resonance imaging (MRI) systems use a magnetic field to sense the structure of an object. The structure creates perturbations and variations in the strength of the magnetic field. The perturbations and variations in the strength of the magnetic field are sensed and interpreted to provide an image of the structure of the object. In many MRI systems, the magnetic field is generated by a superconducting magnet to provide a strong magnetic field.
For a preponderance of superconducting MRI systems, it is necessary to obtain an extremely uniform field in a defined volume because interpreting of the magnetic field assumes a magnetic field of uniform strength. Uniformity is also known as homogeneity. Configuring a plurality of coils which collectively produce an extremely uniform field is possible, but unfortunately when magnets are constructed, manufacturing tolerances introduce minor perturbations in the actual coil dimensions, causing a reduction in the actual homogeneity. In addition, environmental effects such as structural steel in the magnet room can influence the homogeneity.
Overall field homogeneity can be quantified in a number of different ways. The simplest method is to determine the peak to peak variation; the span of a set of points usually plotted over the surface of a sphere. Alternative definitions are surface RMS or volume RMS; the RMS value of a set of points plotted over the surface of a sphere or the RMS value of a set of points plotted (or projected) over the volume of a sphere. The latter produces a figure typically a factor of ten smaller than the peak to peak value.
To further quantify the homogeneity, the variation of the central field of the magnet is usually expressed as a sum of Legendre polynomials and associated Legendre polynomials in a co-ordinate system at which the origin is at the magnet center. The expansion expressed in spherical co-ordinates is shown in Table 1 below:
TABLE 1      H    ⁡          (              r        ,        θ        ,        ϕ            )        =            ∑              n        =        0            ∞        ⁢                  ∑                  m          =          0                          m          =          n                    ⁢                        r          n                ⁢                                  ⁢                                            P              n              m                        ⁡                          (                              cos                ⁢                                                                  ⁢                θ                            )                                ⁢                                          [                                                    A                n                m                            ⁢                              cos                ⁡                                  (                                      m                    ⁢                                                                                  ⁢                    ϕ                                    )                                                      +                                          B                n                m                            ⁢                              sin                ⁡                                  (                                      m                    ⁢                                                                                  ⁢                    ϕ                                    )                                                              ]                    
In table 1, the functions Pnm(cos θ) are known as Associated Legendre polynomials. Anm and Bnm are constants which define the field variation. This is a convenient method of specifying the field variation because reasonably accurate approximations to Legendre polynomials can be produced easily with a series of circular arcs which can be connected together to form a set of correction coils (also referred to as shim coils). A magnet will often be manufactured with several correction coils, where each correction channel is identified by a particular shape of a magnetic field that each correction channel produces.
One conventional means to improve homogeneity of the magnetic field is room temperature correction coils. Room temperature shims are usually manufactured from copper wire and located within the bore of the magnet. However, the extent of capability of room temperature shims to effect homogeneity of the magnetic field is limited. Therefore room temperature shims are usually used to make fine adjustments such as correction of patient susceptibility effects only.
Another conventional means to improve homogeneity of the magnetic field is superconducting correction coils. Superconducting correction coils are secondary coils requiring low temperature for operation and are therefore typically located inside the cryogen reservoir. Although superconducting correction coils are probably the most effective conventional means to improve homogeneity of the magnetic field, superconducting correction coils are also by far the most complex to manufacture. Superconducting correction coils are also susceptible to experiencing large induced currents during a magnet quench. Also, being located within a helium vessel, the superconducting correction coils are totally inaccessible and very difficult to repair. Furthermore, setting currents of the superconducting correction coils requires a high level of technician skill and sophisticated equipment.
Another conventional means to improve homogeneity of the magnetic field is passive shims. Passive shims are usually the least expensive conventional means to improve homogeneity. Passive shims also require a lower level of skill for adjustment among the conventional means to improve homogeneity. However, very large amounts of magnetic material are required to quell large homogeneity perturbations and the large mass of ferrous material can affect performance because of temperature effects of passive shims and white pixels that the passive shims cause. Furthermore, the magnetization of passive shims often makes installation of the passive shims difficult while the primary magnet is energized. Passive shims are also problematic because the passive shims occupy space within the bore of the MRI system and the passive shims require high tray insertion forces. Some manufacturers prefer to avoid passive shims or reduce the quantity of shims by using the passive shims in conjunction with superconducting correction coils.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to improve homogeneity in a magnetic field beyond fine adjustments, that is not very complex to manufacture, install and maintain, that is does not require a large amount of magnetic material.